Encapsulation of polymer based solid state devices with inorganic materials

ABSTRACT

Methods for creating a protective seal suitable for protecting polymer-based electronic devices, including light emitting diodes and polymer emissive displays, are disclosed together with the protected devices. The protective seal includes one or more thin films of silicon nitride (SiN) or other inorganic dielectric applied at low temperature. One or more nonreactive metal layers may be present in the protective layer as well. Other embodiments are disclosed which include a protective cover over the protective layers. These protective layers provide encapsulation with sufficient protection from the atmosphere to enable shelf life and stress life for polymer electronic devices that are adequate for commercial applications.

FIELD OF THE INVENTION

[0001] This invention relates to methods of encapsulating solid stateelectronic devices and the encapsulated devices. More specifically, thisinvention relates to encapsulated organic polymeric light emittingdevices. Principally this invention describes encapsulating such devicesto prevent ambient moisture and oxygen from reacting with materials usedin the fabrication of the devices.

BACKGROUND OF THE INVENTION

[0002] Diodes and particularly light emitting diodes (LED's) fabricatedwith conjugated organic polymer layers have attracted attention due totheir potential for use in display technology [J. H. Burroughs, D. D. C.Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns,and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger,Appl. Phys. Lett. 58, 1982 (1991)]. These references as well as alladditional articles, patents and patent applications referenced hereinare incorporated by reference. Among the promising materials for use asactive layers in polymer LED's are poly (phenylene vinylene), (“PPV”),and soluble derivatives of PPV such as, for example,poly(2-methyoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene),(“MEH-PPV”), a semiconducting polymer with an energy gap E_(g) of ≈2.1eV: This material is described in more detail in U.S. Pat. No.5,189,136. Another material described as useful in this application ispoly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), (“BCHA-PPV”), asemiconducting polymer with an energy gap E_(g) of ≈2.2 eV. Thismaterial is described in more detail in U.S. patent application Ser. No.07/800,555. Other suitable polymers include, for example; OCIC10-PPV;the poly(3-alkylthiophenes) as described by D. Braun, G. Gustafsson, D.McBranch and A. J. Heeger, J. Appl. Phys. 72, 564 (1992) and relatedderivatives as described by M. Berggren, O. Inganas, G. Gustafsson, J.Rasmusson, M. R. Andersson, T. Hjertberg and O. Wennerstrom;poly(paraphenylene as described by G. Grem, G. Leditzky, B. Ullrich, andG. Leising, Adv. Mater. 4, 36 (1992), and its soluble derivatives asdescribed by Z. Yang, I. Sokolik, F. E. Karasz in Macromolecules, 26,1188 (1993), polyquinoline as described by I. D. Parker J. Appl. Phys,Appl. Phys. Lett. 65, 1272 (1994). Blends of conjugated semiconductingpolymers in non-conjugated host polymers are also useful as the activelayers in polymer LED's as described by C. Zhang, H. von Seggern, K.Pakbaz, B. Kraabel, H. -W. Schmidt and A. J. Heeger, Synth. Met., 62, 35(1994). Also useful are blends comprising two or more conjugatedpolymers as described by H. Nishino, G. Yu, T -A Chen, R. D. Rieke andA. J. Heeger, Synth. Met., 48, 243 (1995) Generally, materials for useas active layers in polymer LED's include semiconducting conjugatedpolymers, more specifically semiconducting conjugated polymers whichexhibit photoluminescence, and still more specifically semiconductingconjugated polymers which exhibit photoluminescence and which aresoluble and processible from solution into uniform thin films.

[0003] In the field of organic polymer-based LED's it has been taught inthe art to employ a relatively high work function metal as the anode;said high work function anode serving to inject holes into the otherwisefilled π-band of the semiconducting, luminescent polymer. Relatively lowwork function metals are preferred as the cathode material; said lowwork function cathode serving to inject electrons into the otherwiseempty π*-band of the semiconducting, luminescent polymer. The holesinjected at the anode and the electrons injected at the cathoderecombine radiatively within the active layer and light is emitted. Thecriteria for suitable electrodes are described in detail by I. D.Parker, J. Appl. Phys, 75, 1656 (1994).

[0004] Suitable relatively high work function metals for use as anodematerials are transparent conducting thin films of indium/tin-oxide [H.Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H.Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990); D. Braunand A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)]. Alternatively,thin films of conducting polymers such as poly(aniline), (“PANI”) can beused as demonstrated by G. Gustafsson, Y. Cao, G. M. Treacy, F.Klavetter, N. Colaneri, and A. J. Heeger, Nature, 357, 477 (1992), by Y.Yang and A. J. Heeger, Appl. Phys. Lett 64, 1245 (1994) and U.S. patentapplication Ser. No. 08/205,519, by Y. Yang, E. Westerweele, C. Zhang,P. Smith and A. J. Heeger, J. Appl. Phys. 77, 694 (1995), by J. Gao, A.J. Heeger, J. Y Lee and C. Y Kim, Synth. Met., 82,221 (1996) and by Y.Cao, G. Yu, C Zhang, R. Menon and A. J. Heeger, Appl. Phys. Lett. 70,3191, (1997). Thin films of indium/tin-oxide and thin films ofpolyaniline in the conducting emeraldine salt form are preferredbecause, as transparent electrodes, both enable the emitted light fromthe LED to radiate from the device in useful levels.

[0005] Suitable relatively low work function metals for use as cathodematerials are the alkaline earth metals such as calcium, barium,strontium and rare earth metals such as ytterbium. Alloys of low workfunction metals, such as for example alloys of magnesium in silver andalloys of lithium in aluminum, are also known in prior art (U.S. Pat.Nos. 5,047,687; 5,059,862 and 5,408,109). The thickness of the electroninjection cathode layer has ranged from 200-5000 Å as demonstrated inthe prior art (U.S. Pat. No. 5,151,629, U.S. Pat. No. 5,247,190, U.S.Pat. No. 5,317,169 and J. Kido, H. Shionoya, K. Nagai, Appl. Phys.Lett., 67(1995)2281). A lower limit of 200-500 Ångstrom units (Å) isrequired in order to form a continuous film (full coverage) for cathodelayer (U.S. Pat. No. 5,512,654; J. C. Scott, J. H. Kaufman, P. J. Brock,R. DiPietro, J. Salem and J. A. Goitia, J. Appl. Phys., 79(1996)2745; I.D. Parker, H. H. Kim, Appl. Phys. Lett., 64(1994)1774). In addition togood coverage, thicker cathode layers were believed to provideself-encapsulation to keep oxygen and water vapor away from thechemically active parts of the device.

[0006] Electron-injecting cathodes comprising ultra-thin layer alkalineearth metals, calcium, strontium and barium, have been described forpolymer light emitting diodes with high brightness and high efficiency.Compared to conventional cathodes fabricated from the same metals (andother low work function metals) as films with thickness greater than 200Å, cathodes comprising ultra-thin layer alkaline earth metals withthicknesses less than 100 Å (e.g., 15 Å to 100 Å) provide significantimprovements in stability and operating life to polymer light emittingdiodes [Y. Cao and G. Yu, U.S. patent application Ser. No. 08/872,657.

[0007] Unfortunately, although the use of low work function electrodesis required for efficient injection of electrons from the cathode andfor satisfactory device performance, low work function metals such ascalcium, barium and strontium are typically unstable and readily reactwith oxygen and/or water vapor at room temperature and even morevigorously at elevated temperatures.

[0008] Despite the improvements in the construction of polymer LED's, apersistent problem has been fast decay of the device efficiency (andlight output) during storage and during stress, especially at elevatedtemperature. Thus, there is a need for methods of encapsulation of suchdevices, said encapsulation being sufficient to prevent water vapor andoxygen from diffusing into the device and thereby limiting the usefullifetime.

SUMMARY OF THE INVENTION

[0009] Light-emitting devices fabricated with organic polymericmaterials as the active layers typically comprise reactive low workfunction metals such as, for example, calcium, barium, or strontium.During normal use of these devices, moisture and to a lesser extentoxygen can come in contact with these metals and react to formhydroxides and/or oxides. Exposure to oxygen, particularly in thepresence of light, can lead to photo-oxidative degradation of theluminescent semiconducting polymer as well. Such reactions willsignificantly reduce the performance of the light emitting properties ofthe devices. Prolonged exposure to ambient air leads to significantreduction in light output from devices. Often these reactions will leadto a complete elimination of the light emitting properties of thesedevices, rendering them useless as light sources. Many of the knownprocesses for achieving a hermetic encapsulation of electronic devicesrequire that the devices be heated to temperatures in excess of 300° C.during the encapsulation process. Most polymer based light-emittingdevices are not compatible with such high temperatures.

[0010] We have now found a technique for encapsulating polymericlight-emitting devices at the low method temperatures. The method ofencapsulation provides a hermetic seal between the device and theambient air with its harmful moisture and oxygen.

[0011] The method for encapsulation of this invention is one in whichthe overall thickness of the device is not significantly increased bythe encapsulation of the device.

[0012] The method can, if desired, be carried out with fewer individualprocess steps than methods known to the art.

[0013] In accord with this invention the entire device is protected bydepositing at low temperatures a thin film comprising an inorganicrefractory material, such as a ceramic, for example silicon-nitride orsilicon-oxide over the reactive cathodes present in the devicestructure. In a preferred embodiment, the deposit of the inorganicrefractory material layer is preceded by depositing at low temperaturesa thin film of a non-reactive metal, such as aluminum, over the reactivecathode metal. Following this layer, the thin film comprising aninorganic refractory material, such as a ceramic for examplesilicon-nitride or silicon-oxide is laid down, again at lowtemperatures. The two layer structure is preferred. When depositingthese layers at low temperatures, such as below about 300° C., theytypically contain microscopic pinholes. If the single layer of metalwere used alone as encapsulation, moisture and oxygen would be able topenetrate these pinholes and harm the performance of the device.However, because the probability of a pinhole occurring at exactly thesame location in both layers is insignificant, the use of two layers,the non-reactive metal layer and then the refractory thin film, preventmoisture and oxygen from reaching the reactive materials in the device.This occurs even though the layers are deposited at temperatures below100° C.

[0014] In a preferred embodiment of the invention, the non-reactivemetal layer is patterned in such a way as to form rows across thedevice. This geometry is often used to fabricate matrix displays byforming pixels at the intersections of rows and columns. In thisembodiment the harmful moisture and oxygen can reach the reactivecomponents of the device at the edge of the non-reactive metal rows. Thesubsequently-deposited ceramic film prevents moisture and oxygen fromreaching the reactive metal in this embodiment of the invention.

[0015] In another preferred embodiment the non-reactive metal layer andceramic thin film layer is followed by a thin lid secured by a frame ofepoxy around the perimeter of the device. This lid offers additionalprotection from ambient moisture and oxygen. The lid can be fabricatedfrom any material, which offers a sufficient barrier against moistureand oxygen. Some examples of lids are, plastics, glass, ceramics andother non-reactive metals.

[0016] In yet another preferred embodiment of the invention the lid issecured by dispersing epoxy over a substantial region such as the entirelight-active area of the device.

[0017] In yet another preferred embodiment of the invention the ceramicthin film is patterned into a frame shape. A thin film metal isdeposited on top of this ceramic frame and patterned into the same frameshape. An identically shaped metal thin film is deposited and patternedon the cover plate. The cover plate is attached to the ceramic thin filmframe using metal solder. In this structure the solder and the ceramicthin film frame provide the sealing of the device.

[0018] In yet another preferred embodiment of the invention a ceramicthin film is deposited over the entire active area of the device. A thinfilm metal is deposited and patterned into a frame shape. A similarmetal frame is formed on the cover lid. The cover plate is attached tothe ceramic thin film frame using metal solder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described with referecnebeing made to the accompanyiong drawings in which:

[0020]FIG. 1 shows a schematic cross-sectional view of one embodiment ofa polymer-bassed electronic device protected in accord with the presentinvention.

[0021]FIG. 2 shows a schematic cross-sectional view of anotherembodiment of an electronic device in accord with the invention.

[0022]FIG. 3 shows a schematic cross-sectional view of yet anotherembodiment of the invention.

[0023]FIG. 4 shows a schematic cross-sectional view of yet anotherembodiment of the invention.

[0024]FIG. 5 shows a schematic cross-sectional view of yet anotherembodiment of the invention.

[0025]FIG. 6 shows in plain view, three schematic views of a polymerlight emitting display, freshly made and after two periods of use toillustrate the dregradation in device performance with environmentalexposure in terms of the amount of area of the device actively emittinglight.

[0026]FIG. 7 shows in schematic view a device that has been protectedusing the method of the invention in the configuration of the embodimentshown in FIG. 2. The display shown in FIG. 2 has been exposed for 288hours to a 50° C. temperature and a 95% relative humidity. Notice how nosignificant size reduction of the light emitting areas of the pixels canbe observed in this display even though it has been exposed to muchharsher conditions than the device shown in FIG. 6. It should also benoted that the display in this figure does show a few imperfectionsappearing as incursions into light emitting areas. These “black-spots”do develop over time when the display is exposed to high relativehumidity. These “black-spots are due to imperfections in the thisceramic layer. These imperfections do allow moisture and to a lesserextent oxygen to slowly diffuse trough the layer and react with thereactive metals contained in the cathode metal.

[0027]FIG. 8 shows a perspective view of a display as shown in FIG. 4and as described in example 4 below.

[0028]FIG. 9 shows in schematic view the active area of the device shownin FIG. 8. This particular embodiment is also shown in FIG. 4 above anddescribed in example 4 below. In this case the device has been exposedfor 900 hours to a 50° C. temperature and a 95% relative humidity.Notice how a device packaged with this method shows no reduction in thelight emitting area of the pixels even after prolonged exposure to highrelative humidity. Additionally, it should be noted that no otherimperfections or incursions into the light-emitting areas to such as“black-spots” could be seen on devices packaged with this method.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0029]FIG. 1 shows one embodiment of the invention. FIG. 1 shows across-section of the light-emitting device, which consists of thesubstrate 18 on which a transparent anode layer 16 has been deposited.The anode 16 is followed by one or more polymeric layer(s) 14 and acathode metal layer 12. In the embodiment shown in FIG. 1, the device iscovered with a ceramic thin film protective layer 10 protecting thesensitive cathode metal layer as well as the polymeric layer(s).

[0030]FIG. 2 shows another embodiment of the invention. FIG. 2 shows across-section of the device consisting of the substrate 34 followed bythe anode 30, the polymeric layer(s) 28 and the cathode 26. The ceramicthin film protective layer 24 protects the device. The protective layer24 is enclosed in a cavity, which is filled with an inert gas 21, suchas nitrogen or argon. A cover plate 22 and a perimeter seal of epoxy 20form the cavity.

[0031]FIG. 3 shows yet another embodiment of the invention. FIG. 3 showsa cross-section of the device consisting of the substrate 52 followed bythe anode 50, the light-active polymeric layer(s) 48 and the cathode 46.The device is protected by the ceramic thin film protective layer 44.The protective layer 44 is covered by a layer of epoxy 20 followed by acover plate 42.

[0032]FIG. 4 shows yet another embodiment of the invention. FIG. 4 showsa cross-section of the device consisting of the substrate 76 followed bythe anode 74, the active polymeric layer(s) 72 and the cathode 70. Inthis embodiment of the invention a ceramic thin film 68 is used to forma frame around the air-sensitive components of the device. On top ofthis frame 68 a thin film metal layer 64 is formed in same frame shapeas the ceramic thin film 68. On the cover plate 66 another metalliclayer 60 is formed also in the same shape as the ceramic thin film 68.The cover plate 66 is attached to device using a metallic solder 62around the entire perimeter of the device. To facilitate the solderingprocess a metallic layer 60 is formed on the cover plate. The shape ofthis metallic layer 60 matches the shape of the metallic layer on thedevice 64 and the ceramic thin film frame 68. The soldering is performedin an inert atmosphere so that the cavity formed is filled with an inertgas 78 such as nitrogen or argon.

[0033]FIG. 5 shows yet another embodiment of the invention. FIG. 5 showsa cross-section of the device consisting of the substrate 96 followed bythe anode 94, the polymeric layer(s) 92 and the cathode 90. In thisembodiment of the invention a ceramic thin film 88 is used to as a firstbarrier to protect the device. On top of this ceramic layer 88 a thinfilm metal layer 84 is formed in same frame shape. On the cover plate 86another metallic layer 80 is formed in the same shape as the firstmetallic frame 84. The cover plate 86 is attached to device using ametallic solder 82 around the entire perimeter of the device. Tofacilitate the soldering process a metallic layer 80 is formed on thecover plate. The shape of this metallic layer 80 matches the shape ofthe metallic layer on the device 84. The soldering is performed in aninert atmosphere so that the cavity formed is filled with an inert gas98 such as nitrogen or argon.

[0034]FIG. 6 shows a polymer light emitting display. The same display isshown in three stages; FIG. 6a shows a freshly made device. Thelight-emitting areas are the square areas. FIG. 6b shows an identicaldevice that has been exposed to ambient air, approximately 25° C. and30-40% relative humidity for a period of 24 hours. FIG. 6c shows thesame device after 48 hours of exposure to ambient air at the samecondition. Notice how the light emitting areas of the pixels have beensignificantly reduced after 24 hours. Further notice how the lightemitting areas have almost completely vanished after only 48 hours ofexposure to ambient air. FIG. 6 clearly highlights the importance of aproper packaging technique for polymer light emitting displays.

[0035]FIG. 7 shows a device that has been packaged in the configurationshown in FIG. 2. The display shown in FIG. 2 has been exposed for 288hours to a 50° C. temperature and a 95% relative humidity. Notice how nosignificant size reduction of the light emitting areas of the pixels canbe observed in this display even though it has been exposed to muchharsher conditions than the device shown in FIG. 6. It should also benoted that the display in this figure does show a few imperfectionsappearing as non-light emitting incursions into the light emittingareas. These “black-spots” do develop over time when the display isexposed to high relative humidity. These “black-spots are due toimperfections in the this ceramic layer. These imperfections do allowmoisture and to a lesser extent oxygen to slowly diffuse trough thelayer and react with the reactive metals contained in the cathode metal.

[0036]FIG. 8 shows a perspective view of a display as shown in FIG. 4and as described in example 4 below.

[0037]FIG. 9 shows two magnifications of the active area of the deviceshown in FIG. 8. This particular embodiment is also shown in FIG. 4above and described in example 4 below. In this case the device has beenexposed for 900 hours to a 50° C. temperature and a 95% relativehumidity. The light-emitting areas are the squares with the surroundingareas being non light-emitting. Notice how a device packaged with thismethod shows no reduction in the light emitting area of the pixels asevidenced by their square shape and absence of incursions even afterprolonged exposure to high relative humidity. Additionally it should benoted that no other imperfections such as “black-spots” could be seen ondevices packaged with this method.

[0038] In accord with this invention a ceramic thin film is used toprevent ambient moisture and oxygen from coming into contact with theelectrodes and the polymeric layer(s) of the device which components areelectrically and chemically active. The inorganic refractory material ismade up of one or more oxides and/or nitrides. These materials can betypically selected from full and partial oxides and nitrides of thegroup IIIb and IVb elements. These include the oxides and nitrides ofboron, aluminum, silicon, gallium, germanium, indium, tin, tantalum andlead. Silicon, aluminum, indium and tin are the preferred metals forforming refractory oxides and nitrides, with silicon and aluminum andespecially silicon being most preferred.

[0039] The inorganic refractory layer(s) should be from about 0.025 μmto several (10) microns in thickness with a thicknesses of from 0.05 to5 microns being preferred.

[0040] A cross-section of one embodiment of this structure is shown inFIG. 1. The ceramic layer must be of sufficient integrity to establish ahermetic barrier to moisture and oxygen. Inorganic refractory materialssuch as ceramic materials, for example as silicon-nitride (Si_(x)N_(y)),silicon-monoxide (SiO) or silicon dioxide (SiO₂), can exhibit thenecessary barrier properties if thin films can be formed with sufficientdensity. However, in order to achieve dense films of these types ofmaterials in the past, films must be deposited at elevated temperatures,typically in excess of 400° C. Recently high-density films have beendemonstrated at temperatures below 150° C. using a high-density plasmaduring the film deposition. These lower deposition temperatures haveenabled us to consider the use of ceramic thin films as protectivebarriers in polymer light emitting devices. At these low depositiontemperatures, it is usually not possible to achieve thin films that arecompletely free of microscopic pinholes. However, it is possible toproduce ceramic films with pinhole densities of less than ˜10 pinholesper cm². Since such a pinhole density cannot provide a hermetic seal forthe polymer light emitting devices, it is surprising and unexpected thatby combining these thin ceramic films with a cathode metal structureconsisting of a very thin layer (˜1-100 nm) of a low work functionmetal, such as calcium, barium or strontium, followed by a thicker layer(>100 nm) e.g., 100 to 10,000 nm and especially 100 to 1,000 nm of nonreactive metal, such as aluminum, copper or silver, it is possible toachieve extremely low pin-hole densities (<<0.1 pin-holes/cm²). Althoughboth the ceramic layer and the cathode metal film have pin-holedensities in the range of 1-10 pin-holes/cm², these pin-holes areextremely small typically <<10 μm in diameter. Therefore, theprobability of these pinholes occurring directly on top of each other isextremely low resulting in pinhole density for the entire stack of muchless than 0.1 pin-holes/cm².

[0041] The protective layers are formed using a low temperaturedeposition method. By low temperature is meant a method which depositsthe layers at a substrate temperature of below about 400° C., such asbelow 350° C. Sputtering, including reactive sputtering, may achievethis if the substrate is adequately cooled. Plasma-enhanced chemicalvapor deposition is a preferred method since it achieves a high densityfilm at temperatures of from just above ambient (40° C.) to below 300°C. These methods are known in the art.

[0042]FIG. 6 shows an example of a matrix array of polymer lightemitting pixels before and after exposure to a high temperature highhumidity test. Notice how the pixels without the ceramic thin film arebeing gradually attacked by the moisture eventually completelyeliminating the light output from this device. A similar sample with theceramic thin film barrier is also shown for comparison in FIG. 7. Withthe ceramic thin film barrier, this sample is completely unaffectedunder the same test conditions.

[0043]FIG. 2 shows another embodiment of the invention consisting of thesame structure as described above with an added barrier consisting of acover plate 22 made from ceramic, glass or metallic materials. This lidis attached to device via a perimeter seal of epoxy 20. The purpose ofthe cover plate and epoxy seal is to reduce the requirements on theceramic thin film protective layer by providing an additional barrieragainst moisture and oxygen.

[0044]FIG. 3 shows yet another embodiment of the invention where the lidis attached by completely filling the area between the device and thedevice with epoxy.

[0045]FIG. 4 shows yet another embodiment of the invention. In thisembodiment the lid is attached to device using a metallic solder. Thelid and the solder provide the hermetic seal in this embodiment. Theceramic thin film provides electrical insulation as to prevent shortcircuits between the anode and cathode leads and the solder-seal. Inthis embodiment the ceramic was structured to the shape of the frame asshown in FIG. 4 and FIG. 8. In this embodiment the ceramic layer can bedeposited prior to application of the temperature sensitive polymericmaterials, thus allowing for a wider range of process temperatures.

[0046]FIG. 5 shows yet another embodiment of the invention. In thisembodiment the lid is attached to the device using a metallic solder.The ceramic thin film provides electrical insulation as to prevent shortcircuits between the anode and cathode leads and the solder-seal. Inthis embodiment that solder seal and cover plate provide the primaryprotection and the thin ceramic film provides a secondary barrierprotecting the device.

[0047] The remarkable improvement in stability and lifetime of thepolymer LED's when encapsulated with the methods described in thisinvention is documented in the Examples.

EXAMPLES Example 1

[0048] In this example, a polymer light emitting display consisting ofan array of pixels (30×60) was fabricated. The fabrication of theDisplay required several steps. First, the anode layer, consisting ofIndium Tin Oxide (ITO), was patterned on the glass substrate intocolumns; in this Example 60 columns were formed. The entire device wasthen coated with a light emitting polymer material. Examples includedOC1C10-PPV and MEH-PPV and related soluble PPV derivatives. Next thecathode metal, consisting of a thin layer of a low work-function metalwas deposited, followed by deposition of a layer of Aluminum (theAluminum layer was added simply to protect the more reactive Ca layer).The cathode layer was patterned in such a way as to form rows; said rowswere oriented perpendicular to the underlying anode columns. In thisexample 30 rows were formed. Light emitting pixels were formed, thereby,at each column-row intersection. Hence the display in this exampleconsisted of 1800 pixels. In order to prevent oxygen and moisture fromthe ambient air from reacting with the low work-function metal in thecathode, the entire device was coated with a thin layer (˜1 micron) ofsilicon-nitride. The coating was performed using Plasma EnhancedChemical Vapor Deposition (PECVD). By utilizing a high-density plasma,this deposition was accomplished with the display at a temperature ofonly 85° C. Exposure of the display to this relatively low temperaturecaused no significant damage, yet a thin film of silicon-nitride with alow pinhole density was formed. This thin film of silicon-nitridetogether with the protective layer of Aluminum forms a near hermeticseal preventing oxygen and moisture from the ambient air from reachingthe reactive metal underneath the Aluminum layer. A cross section of adevice as described in this Example is shown in FIG. 1. FIG. 6 shows thedegradation of pixels in an unprotected device when exposed to highhumidity. FIG. 7 shows a device sealed as described in this Example.Note how no significant degradation can be seen at the horizontal edgesof the light emitting pixels in the silicon-nitride coated device.

Example 2

[0049] In this Example, a polymer emissive display was fabricated asdescribed in Example 1. Following the deposition of silicon-nitridelayer a secondary lid was applied. This lid consisted of a thin (0.7 mm)glass plate. The lid was attached to the device using a perimeter sealof epoxy. The epoxy seal was located outside the perimeter of thesilicon-nitride layer. The sealing was performed in an inert gasenvironment, a controlled atmosphere dry box containing, argon gas(alternatively, nitrogen gas was also be used). The purpose of thissecondary seal is to further increase the lifetime of the device byincreasing the time it takes for any moisture in the ambient air toreach the reactive low work-function metal in the cathode of thedisplay. Any moisture from the ambient air must first penetrate theepoxy seal and then diffuse through any pinhole or imperfection in thesilicon-nitride layer. A cross-section of this type of device is shownin FIG. 2.

Example 3

[0050] In this Example, a polymer emissive display was fabricated asdescribed in Example 1. Following the deposition of silicon-nitridelayer a secondary lid was applied. This lid consisted of a thin (0.7 mm)glass plate. The lid was attached to the device using an evenlydistributed layer of epoxy. The epoxy seal was located outside theperimeter of the silicon-nitride layer. The sealing was performed in aninert gas environment, a controlled atmosphere dry box containing, argongas (alternatively, nitrogen gas was also be used). The purpose of thissecondary seal is to further increase the lifetime of the device byincreasing the time it takes for any moisture in the ambient air toreach the reactive low work-function metal in the cathode of thedisplay. Any moisture from the ambient air must first diffuse throughthe epoxy seal and then diffuse through any pinhole or imperfection inthe silicon-nitride layer. A cross-section of this type of device isshown in FIG. 3.

Example 4

[0051] This Example involves a polymer emissive display, which wassimilar to the device described in Example 1-3. However, the processsequence was changed to allow for a wider process window during thesilicon-nitride deposition. In this Example, the anode columns wereformed as in Example 1. Following the patterning of the anode, a thinlayer of silicon-nitride was deposited. The silicon-nitride layer wasstructured as to form a frame surrounding the active area of thedisplay. A thin layer of metal was then deposited on top of thesilicon-nitride frame. Next the light emitting polymer layer wasdeposited followed by the cathode which was patterned into rows, asdescribed in Example 1 above. A separate glass lid with a matching metalframe was fabricated and attached to the display using a low meltingpoint solder. In this Example, the glass-lid and the metal-solder frameproduced the seal. The silicon-nitride layer provided electricalinsulation, which prevented the solder from shorting the anode andcathode lines. A cross-section of this device is shown in FIG. 4. Aphoto of this device is shown in FIG. 8. A photo of a device afterextensive exposure to high humidity conditions is shown in FIG. 9. Notethe complete absence of black spots as well as no observable reductionin emitted light at the edges of the pixels.

Example 5

[0052] In this example, a polymer emissive display was constructed asdescribed in Example 1 above. Following the silicon-nitride deposition,a metal frame was deposited around the perimeter of the silicon-nitridelayer. A separate glass lid was fabricated also with a metal frame thedimensions of which matched the frame on the silicon-nitride layer insize and shape. The glass lid was subsequently attached to the displayusing a low melting point solder (135° C., in this Example). In thiscase the primary protection of the display came from the glass lid andits associated metal solder seal. The purpose of the silicon-nitride inthis Example was to prevent the metal frame seal from creatingelectrical shorts between the columns forming the anode and the rowsforming the cathode. A cross-section of this device is shown in FIG. 5.

We claim:
 1. In a light-emitting device comprising a layer of an active light-emitting polymer sandwiched between a cathode and an anode, the improvement comprising an encapsulating layer of low-temperature-applied inorganic material protecting the device against environmental attack.
 2. The device of claim 1 wherein the low-temperature-applied inorganic material comprise an oxide or nitride of a group IIIb or group IVb element.
 3. The device of claim 2 wherein the inorganic material is a silicon-based material.
 4. The device of claim 3 wherein the inorganic material is a silicon-based material selected from silicon-nitride and silicon-oxide.
 5. The device of claim 1 additionally comprising a low-temperature-applied coating comprising a nonreactive metal located between the device and the layer of inorganic material.
 6. The device of claim 1 additionally comprising a protective cover plate attached over the layer of inorganic material.
 7. The device of claim 6 wherein the protective cover plate is attached with epoxy.
 8. The device of claim 7 wherein the epoxy is around the perimeter of the cover.
 9. The device of claim 8 additionally comprising an inert gas between the cover and the layer of inorganic material.
 10. The device of claim 7 wherein the epoxy is uniformly distributed between the cover and the layer of inorganic material.
 11. The device of claim 5 additionally comprising a protective cover plate attached over the layer of inorganic material.
 12. The device of claim 11 wherein the protective cover plate is attached with epoxy.
 13. The device of claim 12 wherein the epoxy is around the perimeter of the cover.
 14. The device of claim 13 additionally comprising an inert gas between the cover and the layer of inorganic material.
 15. The device of claim 12 wherein the epoxy is uniformly distributed between the cover and the layer of inorganic material.
 16. The device of claim 6 wherein the protective cover plate is attached with solder.
 17. The device of claim 11 wherein the protective cover plate is attached with solder.
 18. The device of claim 1 wherein the coating of inorganic material is in the form of a frame plate.
 19. The device of claim 1 wherein the layer of inorganic material is applied at a temperature below 400° C.
 20. The device of claim 5 wherein the nonreactive metal is applied at a temperature below 400° C.
 21. The device of claim 5 wherein both the nonreactive metal and the inorganic material are applied at a temperature below 400° C.
 22. A method for protecting a light-emitting device comprising an active light-emitting polymer sandwiched between a cathode and an anode comprising encapsulating said device with an encapsulating layer of low-temperature-applied inorganic material.
 23. The method of claim 22 wherein the low-temperature-applied inorganic material comprise an oxide or nitride of a group IIIb or group IVb element.
 24. The method of claim 22 additionally comprising the step of overcoating the layer of low-temperature-applied inorganic material with a layer of nonreactive metal.
 25. The method of claim 22 additionally comprising the step of attaching a protective plate over the layer of inorganic material.
 26. The method of claim 24 additionally comprising a low-temperature-applied coating comprising a nonreactive metal located between the device and the layer of inorganic material. 